DDS Loop Technology and Coding Format
Dataphone Digital Service (DDS) is a private line digital data point-to-point, or point-to-multipoint communication service, widely deployed in the United States of America and is described in detail in the following references (incorporated herein by reference):
1. "D4 Digital Channel Bank Family", Bell System Technical Journal, Vol. 61, Number 9, Part 3, November 1982. PA1 2. "Digital Data System", Bell System Technical Journal, Vol. 54, Number 5, May-June 1975. PA1 3 . "Generic Requirements for the Subrate Multiplexer", Issue 1, TA-TSY-000189, Bell Communications Research Inc., April 1986. PA1 4. "Digital Data System Requirements for the Office Channel Unit", Issue 1 TA-TSY-000083, Bell Communications Research Inc., December 1984. PA1 5. "Digital Data System--Multipoint Junction Unit Requirements" Issue 2 TA-TSY-000192, Bell Communications Research Inc., April 1986. PA1 6. "Carrier to Customer Installation--DS1 Metallic Interface" ANSI T1, 403-1989.
The data rate at which a DDS customer can obtain service are the subrates 2.4 kbps, 4.8 kbps, 9.6 kbps, 19.2 kbps, 38.4 kbps and the full rates 56 kbps, 64 kbps. The local distribution of a DDS connection uses metallic, twisted-pair cables for the full-duplex four-wire transmission path between the customer premises and the serving DDS office. A Channel Service Unit (CSU) serves to terminate the four-wire loop at the customer premises. At the DDS serving office the loop is terminated with an Office Channel Unit (OCU). The OCU encodes the incoming data signals into an 8-bit byte format that adds necessary control information and, regardless of the data service rate, builds the signal up to a rate of 64 kbps.
The DDS local loop signalling format employs Alternate Mark Inversion (AMI) encoding to convert digital data signals generated at the customer premises to an AMI line code format. In AMI, a ONE bit is transmitted as a pulse transition (polarity change) and a ZERO bit as no pulse. The digital signal on the local loop is baseband, bipolar, return-to-zero, with 50-percent duty cycle. This signal has a symbol rate equal to the data rate. A binary 1 is transmitted as a positive or negative pulse, in opposite polarity to the preceding pulse. A binary 0 is represented by the absence of a pulse. The local loop signal format is described in detail in the following reference (incorporated herein by reference):
As shown in FIGS. 1a and 1b, the loop signal between the CPE 10 and the OCU 12 (with non-secondary channel information) is formatted in 6 bit bytes containing six channel data bits D1-D6 for the data rates of 2.4, 4.8, 9.6, 19.2 and 38.4 kpbs (FIG. 1a), or 7 bit bytes containing seven channel data bits D1-D7 for 56 kbps (FIG. 1b). The presence of network control information is indicated by modifying the standard bipolar signal. The signal is modified by inserting a violation `V` pulse into the bit stream. This pulse has the same polarity as the immediately preceding pulse (hereinafter "the "previous" pulse), thus violating the standard format.
Unrestricted insertion of violations would produce an undesirable dc component on the local loop. Therefore, a bit period is reserved two bits prior to a violation, for insertion of a bipolar pulse or no pulse, i.e., a zero, in such a manner that successive violations alternate in polarity. Calling this inserted pulse an `X` bit; modified bipolar signals are shown in FIGS. 2a and 2b with X bit values of 0 and 1. If the number of pulses since the previous violation is odd, the X bit is a zero. If the number of pulses since the last violation is even, a pulse of opposite polarity to the previous pulse is inserted into the pulse stream as the X bit. The complete violation sequence includes a forced zero between the X and V bits; therefore, the sequence is called an XOV sequence.
A secondary channel capability has been proposed to offer a companion digital transmission channel independent of the primary channel and at a lower rate. Secondary channel capability requires that the loop signal be structured so that the primary and secondary channel information can be differentiated. As shown in FIGS. 3a and 3b, the loop signal with secondary channel information is formatted in 8 bit bytes containing six primary channel data (D) bits D1-D6 for the primary channel rates of 2.4, 4.8, 9.6, 19.2 and 38.4 kbps (FIG. 3a), or 9 bit bytes containing 7 D bits for 56 kbps primary channel data, or 9 bit bytes containing 8 D bits for 64 kbps primary channel data (FIG. 3b). Each byte contains an "F" bit for framing, and a "C" bit arising out of the substitution of the secondary channel information on no more than one out of every three C bits (FIG. 3c). Intentional Bipolar Violations (BPV) in the loop signal with secondary channel information are not required. BPVs are used in the basic DDS service to transmit control and supervisory information.